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Syscheck

Syscheck is the name of the integrity checking process inside OSSEC. It runs periodically to check if any configured file (or registry entry on Windows) has changed.

Why Integrity checking?

This is the explanation from the OSSEC book:

There are multiple types of attacks and many attack vectors, but there is one thing unique about all of them: they leave traces and always change the system in some way. From viruses that modify a few files, to kernel-level rootkits that alters the kernel, there is always some change in the integrity of the system.

Integrity checking is an essential part of intrusion detection, that detects changes in the integrity of the system. OSSEC does that by looking for changes in the MD5/SHA1 checksums of the key files in the system and on the Windows registry.

The way it works is that the agent scans the system every few hours (user defined) and send all the checksums to the server. The server stores the checksums and look for modifications on them. An alert is sent if anything changes.

Quick facts

  • How often does it run?
    • By default every 6 hours, but the frequency or time/day are configurable.
  • Where is the database stored?
    • On the manager in /var/ossec/queue/syscheck.
  • How does it help with compliance? (PCI DSS, etc)
    • It helps with sections 11.5 (install FIM software) and 10.5 (integrity checking of log files) of PCI.
  • How much CPU does it use?
    • The scans are performed slowly to avoid using too much CPU/memory.
  • How are false positives handled?
    • Files can be ignored manually in the configuration or using rules. By default when a file has changed 3 times further changes are automatically ignored.

Realtime options

ossec-syscheckd is able to check file integrity in near realtime on Windows and modern Linux distros. Windows comes with support out of the box, but on Linux systems inotify packages may need to be installed. Check for inotify dev packages, and possibly an inotify-tools package.

Configuration options

These configuration options can be specified in each agent's ossec.conf file, except for the auto_ignore and alert_new_file which apply to manager and local installs. The ignore option applies to all agents if specified on the manager.

Configuration Examples

To configure syscheck, a list of files and directories must be provided. The check_all option checks md5, sha1, owner, and permissions of the file.

Example:

<syscheck>
    <directories check_all="yes">/etc,/usr/bin,/usr/sbin</directories>
    <directories check_all="yes">/root/users.txt,/bsd,/root/db.html</directories>
</syscheck>

Files and directories can be ignored using the ignore option (or registry_ignore for Windows registry entries):

<syscheck>
    <ignore>/etc/random-seed</ignore>
    <ignore>/root/dir</ignore>
    <ignore type="sregex">.log$|.tmp</ignore>
</syscheck>

The type attribute can be set to sregex to specify a :ref:`regex` in the ignore option.

<syscheck>
    <ignore type="sregex">^/opt/application/log</ignore>
</syscheck>

A local rule can be used to modify the severity for changes to specific files or directories:

<rule id="100345" level="12">
    <if_matched_group>syscheck</if_matched_group>
    <match>/var/www/htdocs</match>
    <description>Changes to /var/www/htdocs - Critical file!</description>
</rule>

In the above example, a rule was created to alert with high severity (12) for changes to the files in the htdocs directory.

Real time Monitoring

OSSEC supports realtime (continuous) file integrity monitoring on Linux (support was added kernel version 2.6.13) and Windows systems.

The configuration is very simple. In the <directories> option where you specify what directories to monitor, adding realtime="yes" will enable it. For example:

<syscheck>
    <directories realtime="yes" check_all="yes">/etc,/usr/bin,/usr/sbin</directories>
    <directories check_all="yes">/bin,/sbin</directories>
</syscheck>

In this case, the directories /etc, /usr/bin and /usr/sbin will be monitored in real time. The same applies to Windows too.

Warning

The real time monitoring will not start immediately. First ossec-syscheckd needs to scan the file system and add each sub-directory to the realtime queue. It can take up to 30 minutes for this to finish (wait for the log "ossec-syscheckd: INFO: Starting real time file monitoring" ).

Note

Real time only works with directories, not individual files. So you can monitor the /etc or C:\program files directory, but not an individual file like /etc/file.txt.

Note

Both rootcheck and syscheck runs on the same thread, so when rootcheck is running, the inotify events would get queued until it finishes.

Report Changes

OSSEC supports sending diffs when changes are made to text files on Linux and unix systems.

Configuring syscheck to show diffs is simple, add report_changes="yes" to the <directories option.

For example:

<syscheck>
    <directories report_changes="yes" check_all="yes">/etc</directories>
    <directories check_all="yes">/bin,/sbin</directories>
</syscheck>
.. notes::

    Report Changes will only work with text files, and the changes are stored on the agent
    inside ``/var/ossec/queue/diff/local/dir/file``.